The present invention relates to a method of refining a cyclic ester. The cyclic ester is a condensation product of a dicarboxylic acid and diol, or a product made from a hydroxycarboxylic acid dimer which is made circular by dehydrate-condensation. The cyclic ester is used as a starting material for producing a polyester or polyester amide including a biodegraded polymer or medical-use polymer.
It is well known that the cyclic ester is produced by depolymerizing hydroxycarboxylic acid oligomer. There are two types of crude cyclic esters produced by the conventional method. One is a cyclic ester, wherein no solvent is adhered to its crystal, and the other is a cyclic ester, wherein a high-boiling point organic solvent is adhered to its crystal. The crude cyclic esters are refined by the appropriate method for each of the crude cyclic esters.
The following references describe the manufacturing method for the first-type cyclic ester.
U.S. Pat. No. 2,668,162 describes that a glycolic acid oligomer is reduced to powder and supplied slowly to a reaction vessel, in which it is depolymerized by heating under a super vacuum. Then, the produced gas is cooled for solidification into a crude cyclic ester.
U.S. Pat. No. 4,727,163 describes that a large amount of polyether is block-copolymerized with a small amount of glycolic acid to make a copolymer. The copolymer is heated under a low pressure to be depolymerized. Then, the produced gas is cooled for solidification into the crude cyclic ester.
U.S. Pat. No. 4,835,293 describes that a glycolic acid oligomer is heated into a liquid. The liquid is provided with a flow of nitrogen to increase the surface area so that the liquid is evaporated from the increased surface area. The evaporated gas is provided with a flow of nitrogen and cooled for solidification into a crude cyclic ester.
U.S. Pat. No. 5,326,887 and WO 92/15572A1 describe that a glycolic acid oligomer is heated and depolymerized over a fixed bed catalyst system. Then, produced gas is cooled and solidified into a crude cyclic ester.
The crude cyclic esters produced by the above methods contain impurities such as a hydroxycarboxylic acid oligomer and hydroxycarboxylic acid itself. The crude cyclic esters are purified by re-crystallization using a solvent such as an isopropanol disclosed by European Patent No. 261572, a t-amyl alcohol disclosed by German Patent Publication No. 1808939, a carbon tetrachloride disclosed by German Patent Publication No. 123473904, an ethyl acetate disclosed by U.S. Pat. No. 4,727,163, and an ether disclosed by Japanese Kokai No. 06-172341. A slurry of the crystal produced by the re-crystallization is separated into a solid and a liquid by filtering, cleaned by the solvent used in the re-crystallization process or an other cleaning liquid, and dried to remove the solvent or cleaning liquid, thus providing a purified crystal.
Japanese Kokai No. 09-328481 describes a method of manufacturing the second-type crude cyclic ester, in which a mixture comprising an hydroxycarboxylic acid oligomer and at least one kind of high-boiling point organic solvent having a boiling point between 230xc2x0 C. and 450xc2x0 C. is heated under normal or reduced pressure at such a temperature as to depolymerize the oligomer. The depolymerized oligomer is dissolved in the solvent until the remaining rate of the oligomer in the liquid phase reaches 0.5% or less. The heating is further continued at the above temperature to depolymerize the remaining oligomer. The produced cyclic ester is distilled out with the high-boiling point organic solvent, and the crude cyclic ester is recovered from the distilled product.
By the method, the cyclic ester is distilled with at least one organic solvent having a boiling point of 230-450xc2x0 C., and the crude cyclic ester is collected by cooling the distilled product. In this case, a non-solvent may be added in the distilled product, and the solidified or crystallized cyclic ester is separated into a solid and a liquid. The thus produced crystal of the cyclic ester includes the high-boiling point organic solvent attached thereto, which is difficult to remove by the usual drying method. However, it is indispensable to remove the high-boiling point organic solvent by any method in order to obtain a dried crystal.
A conventional method to remove such a liquid as adhered to the crystal of the cyclic ester is to displace and clean the crystal with a low-boiling point cleaning liquid, such as cyclohexane or ether, and then, remove the low-boiling point cleaning liquid by drying. The drying process is performed at a temperature lower than the melting point of the crystal. Since the cyclic ester sublimates, the loss of the crystal is increased if the pressure is reduced excessively during the drying process. In addition, if necessary, the re-crystallization is performed using an ethyl acetate. Even in this case, the adhered solvent has to be removed by drying. As fully stated above, in the conventional method using the high-boiling point organic solvent, the low-boiling point cleaning liquid is indispensable to displace the liquid adhered to the crystal. As a result, the cleaning liquid waste is a mixture comprising the high-boiling point organic solvent and the low-boiling point cleaning liquid.
The method of refining the first-type of crude cyclic ester requires additional and complicated processes including the processes for drying to remove the solvent or cleaning liquid from the crystal surface, cooling to collect the solvent or cleaning liquid removed during the drying process, evaporating to separate the mixture of the solvent and the cleaning liquid collected during the cooling process. The drying process is performed at a temperature lower than the melting point of the crystal. Also, since the cyclic ester sublimates, the loss of the crystal is increased under excessively reduced pressure. A solvent, such as an alcohol, may bring about ester exchange reaction with the cyclic ester, and re-crystallization is required a few times to remove the impurities caught inside the crystal.
The method of refining the second-type of crude cyclic ester also requires complicated processes including the processes for drying to remove the cleaning liquid adhered to the crystal surface, collecting the cleaning liquid removed during the drying process, and disposing to refine and collect a mixture of the high-boiling point organic solvent and low-boiling point cleaning liquid. A solvent, such as an alcohol, may bring about ester exchange reaction with the cyclic ester, and re-crystallization is required a few times to remove the impurities caught inside the crystal.
Accordingly, it is an object of the present invention to provide a method of refining the crystal of a crude cyclic ester, which is simple and applicable to both the first- and second-types of crude cyclic esters.
According to the present invention, crystal are refined by means of a vertically extending cylindrical refiner provided with an inlet for supplying crude cyclic ester at a lower part of the refiner, and provided with an outlet for taking crystals as a product at an upper part of the refiner, and provided with an agitator for agitating crude cyclic ester in the refiner
According to the present invention, a method of refining crystals of a crude cyclic ester comprises the steps of supplying the crude cyclic ester to a vertically extending cylindrical refiner from an inlet provided at a lower part of the refiner, agitating the crude cyclic ester by an agitator provided in the refiner to make the crude cyclic ester flow upwardly, refining the crystals of the crude cyclic ester by counterflow-contact between the upflowing crude cyclic ester and a downflowing melted liquid containing a refined crystal component, and taking out refined crystals of the crude cyclic ester from an outlet provided at an upper part of the refiner.
According to the invention, the crystal of the crude cyclic ester is refined by the counterflow-contact between the upflowing crude cyclic ester and the downflowing melted liquid containing the refined crystal component in the refiner. That is, inventors have found a method that a mother liquid and impurities adhered to the crystal are cleaned off, and those that are taken inside the crystal are sweat off by the counterflow-contact.